A preferred bidder for the biggest ever public procurement contract - potentially worth £17bn and considerably larger than the Olympic games - is to be announced today, kickstarting a move of control for the massive Sellafield nuclear site in Cumbria from the state to the private sector.

Unions have expressed concerns that the change of operator may lead to a major cost-cutting drive which could bring job losses to some of the 10,000 workers, while Greenpeace warns that corners could be cut on safety to increase profits.

Four US-dominated companies or consortiums - only two of which have any real UK component - are in the running with CH2M Hill competing against Bechtel, Washington Group and Fluor to operate troubled reprocessing facilities such as Thorp and Mox as well as cleaning up a power station on the site, Calder Hall.

There has been repeated speculation that CH2M Hill, the only firm bidding on its own, will win the £1bn-a-year deal, which will run for an initial five years and which could extend to 17. However the Nuclear Decommissioning Authority (NDA) insisted yesterday that none of the speculation had been based on concrete information.

"Obviously you have a one-in-four chance of guessing the right name, but only four people in the competition team know who the preferred bidder is, and they will reveal that name on Friday morning. All four bidders are of equal status and all four passed the threshold test," said an NDA spokesman.

The secretary of state for business, John Hutton, will first have to endorse the choice before it is simultaneously rung through to all four bidders who have been told to be on hand close to Sellafield. The victorious private executive will then be driven by car to a formal meeting with Ian Roxburgh, NDA chief executive, at the Sellafield Centre.

The NDA expects to be able to confirm the final contract in October, when the private consortiums would take over as Sellafield's new parent body organisation leading to the final dismantling of BNFL, a holding company that has been gradually hiving off all its functions.

The site is currently operated by Sellafield Ltd, an arm of state-owned BNFL. It is owned by the NDA, and this is not set to change.

The full list of the companies waiting last night for news of the Sellafield contract are CH2M Hill; Bechtel - in a consortium with Serco and Babcock & Wilcox; URS Washington Group with Amec and Areva of France; and Fluor with Toshiba, the Japanese buyer of the former BNFL's nuclear design business, Westinghouse.

The parent body will not "own" Sellafield but will be paid some £1bn a year and its profits will come from squeezing efficiencies out of the work programme.

The Unite union is concerned that this could be done through redundancies and is aware that a report five years ago warned that 8,000 jobs could go within five years.

The proposed new operator will be announced at a difficult time for Europe's biggest industrial complex, where workers have rejected a 2% pay offer and are to be balloted on industrial action.

Greenpeace voiced concern about the prospect of an overseas firm winning the contract, warning that it could be difficult to access information about its activities.

Jean McSorley, a campaigner, said today's announcement could also have "massive" implications for the taxpayer, adding: "The history of decommissioning and clean-up has been bedevilled by huge cost overruns in the past. There are also issues over the environment and public safety if a private firm wants to increase profits and improve efficiency."

A House of Commons committee of public accounts said in a report issued yesterday that the cost of the clean-up was soaring, with little sign that the latest figure on final costs - £73bn - could not go any higher.